Method of tuning a summing network

ABSTRACT

The present invention relates to a summing network comprising: a summing point (P) having interfaces for coupling summing network branches to the summing point, and an interface for coupling the summing point (P) to an antenna (ANT), and channel units (TX 1  to TX 3 ) arranged in the branches, and channel unit-specific band-pass filters ( 1  to  3 ), whose pass frequencies correspond to the frequency or frequencies of the corresponding channel unit. To facilitate tuning the summing network, a compensation element ( 7 ) is coupled to the summing network between the summing point (P) and the antenna (ANT), the load effect of said compensation element corresponding substantially to the load caused to the summing point (P) by the branches coupled to the summing point.

[0001] The present invention relates to a summing network andparticularly to dimensioning a summing network to optimize performance.

[0002] A summing network is used, e.g., in base stations in mobilesystems for combining the base station's transmitter branches to acommon transmission antenna. In the following, the invention will bedescribed by way of example by specifically referring to the summingnetwork of a base station, although the invention is also applicable toother summing networks.

[0003] In a base station summing network, each transmitter branchcomprises a transmitter and a narrowband band-pass filter, whose passfrequency corresponds to the transmission frequency used by thetransmitter. Band-pass filters, i.e. combiner filters, preventtransmitters from interfering with each other's functioning. Inpractice, each band-pass filter is usually tuned to the intermediatefrequency of the corresponding transmitter in such a way that it passesthe signal transmitted by the corresponding transmitter with a minimumloss to the summing network, and simultaneously prevents the signals ofother transmitter at different frequencies from passing to thecorresponding transmitter.

[0004] In order for a maximally large portion of the transmission powerof the transmitters to be transferred to an antenna, the summing networkhas to be tuned with respect to the frequency channels used by the basestation transmitters. The optimal electrical length of a summing networkdepends on the wavelength of the carrier of the signal to betransmitted. Strictly taken, a summing network is thus tuned only at onefrequency. A summing network is generally tuned to the middle of theavailable frequency band, i.e. the M-frequency. In this case, the cablesof the summing network with which the band-pass filters of thetransmitter branches are coupled to the summing point are usuallyselected so that their length is λ/2, wherein λ is the wavelength at theM-frequency.

[0005] When moving away from the optimum frequency, i.e. usually theM-frequency, to the lower end of the available frequency band to theB-frequency or to the upper end to the T-frequency, the electricallengths of the cables used no longer correspond to the value λ/2, i.e.the electrical length of the cables is wrong. This causes load to thesumming point, i.e. reactive mismatching. This load causes impairedreflection loss and pass loss, as well as narrowing of the bandwidths ofthe combiner filters.

[0006] In practice, the optimal frequency band of a summing network istoo narrow to allow the frequency channels of the base stationtransmitters to be changed very much without having to deal with thetuning of the summing network. However, in practice, the frequencychannels of base stations in mobile systems, for example, need to bechanged between the B-frequency and the T-frequency. Automatically (byremote control) tuneable combiner filters becoming common, the needarises to facilitate the tuning of a summing network. A previously knownsolution, wherein an engineer visits the site of the base station andreplaces the cabling of the summing network with cabling tuned to a newfrequency band, is a too expensive and time consuming task.

[0007] Solutions are also previously known, wherein the summing networkis provided with remotely controlled adjusting elements enabling there-tuning of the summing network. However, these adjusting elements arerelatively complex, and their manufacture and management increase costs.

[0008] The object of the present invention is to solve the above problemand to provide a solution that facilitates the tuning of a summingnetwork. This object is achieved by the method of tuning a summingnetwork according to the invention. The method of the invention ischaracterized by determining the load caused to a summing point bysumming network branches, selecting a compensation element whose loadeffect corresponds substantially to the load caused to the summing pointby the summing network branches, and coupling said compensation elementto the summing network between the summing point and an antenna.

[0009] If the summing point is coupled via a conductor directly to theantenna, the compensation element can be coupled between the summingpoint and the antenna by coupling it to a conductor connecting them.However, if the summing point in a summing network is coupled to theantenna via another component (or other components), the compensationelement is coupled between the summing point and the following componentto a conductor connecting them. The term conductor refers to a transferline for transferring signals between components. It may thus be e.g. acoaxial cable, a microstrip conductor or the like.

[0010] The invention also relates to a summing network comprising: asumming point having interfaces for coupling summing network branches tothe summing point, and an interface for coupling the summing point to anantenna, and channel units arranged in the branches, and channelunit-specific band-pass filters, whose pass frequencies correspond tothe frequency or frequencies of the corresponding channel unit. Thesumming network of the invention is characterized in that a compensationelement is coupled to the summing network between the summing point andthe antenna, the load effect of said compensation element correspondingsubstantially to the load caused to the summing point by the branchescoupled to the summing point.

[0011] The invention is based on the idea of arranging a compensationelement, whose load effect is substantially the same as the load causedto the summing point by the summing network branches, in the summingnetwork between the summing point and the antenna. The use of such acompensation element eliminates the need to retune the summing networkwhen the frequency channels of the channel units are changed. This isbecause the compensation element compensates for the mismatching causedby the electrical length of the summing network branches no longer beingoptimal in the new frequency band.

[0012] In this context, the concept channel unit refers to atransmitter, a receiver or a combination thereof. The solution of theinvention is applicable for use both in a summing network intransmission use and in a summing network in reception use. A summingnetwork in transmission use is used to sum the transmitter signals andapply them to a common transmission antenna. A summing network inreception use is used to branch signals received with the common antennato the different receivers. In accordance with the invention, thesumming point may be coupled either directly with a conductor to theantenna or, alternatively, via another component (or other components)to the antenna. An alternative is for the summing point to be coupled tothe antenna via another summing point.

[0013] The most significant advantage of the method and summing networkof the invention is thus that the cabling connecting the summing networkbranches to the summing point does not have to be replaced and thesumming network does not need any other kind of tuning even if thefrequencies of the channel units in the base station are changed.

[0014] In accordance with the invention, at its simplest, thecompensation element could be a conductor whose length correspondssubstantially to the total length of the conductors connecting thepass-band filters to the summing point. A significant additionaladvantage achieved by such a solution is a temperature-compensatedsumming network. This is because changes due to the change in thetemperature of the conductor connecting the branches to the summingpoint and the conductor of the compensation element compensate for eachother.

[0015] If the pass loss of the summing network is to be less than isachieved when a conductor is used as the compensation element, thecompensation element can be a resonator, comprising e.g. a capacitor anda coil. In this case, the first poles of the capacitor and the coil arecoupled to the connector between the summing point and the antenna, andthe second poles are grounded.

[0016] The preferred embodiments of the invention are disclosed in theattached dependent claims 2 to 3 and 5 to 12.

[0017] In the following, the invention will be described by way ofexample with reference to the attached figures, of which

[0018]FIG. 1 is a block diagram of a first preferred embodiment of thesumming network of the invention,

[0019]FIG. 2 is a block diagram of a second preferred embodiment of thesumming network of the invention,

[0020]FIG. 3 is a block diagram of a third preferred embodiment of thesumming network of the invention,

[0021]FIG. 4 is a flow diagram of a first preferred embodiment of themethod of the invention,

[0022]FIG. 5 is a block diagram of a fourth preferred embodiment of thesumming network of the invention, and

[0023]FIG. 6 is a block diagram of a fifth preferred embodiment of thesumming network of the invention.

[0024]FIG. 1 is a block diagram of a first preferred embodiment of thesumming network of the invention. The summing network in FIG. 1 may bee.g. a summing network of a base station in a mobile system, the networkserving as means to couple channel units TX1 to TX3 of different summingnetwork branches to a common antenna ANT. The channel units TX1 to TX3may be receivers, transmitters or combinations thereof. However, in thefollowing the assumption is by way of example that the channel units TX1to TX3 are transmitters whose signals are combined by the summingnetwork and applied to a common transmission antenna.

[0025] In the case of FIG. 1, the summing network of a base stationcomprises three branches each including a channel unit TX1 to TX3 and aband-pass filter 1 to 3. In practice, the number of branches maynaturally deviate from the exemplary case of FIG. 1, which includesthree of them. The pass frequency of each band-pass filter 1 to 3 isadjusted to correspond to the frequency or frequencies used by thecorresponding channel unit. This means that e.g. band-pass filter 1passes the transmission-frequency signals produced by channel unit TX1to the summing network, while it simultaneously prevents the signalsproduced by the other channel units TX2 and TX3 from passing to channelunit TX1. Band-pass filters 1 to 3 are connected to summing point P withconductors 4 to 6. In this context, conductors refer to transfer linesfor transferring signals from one component to another, i.e. coaxialcables or microstrip conductors, for example. At summing point P, thesignals produced by the different transmitters are combined and appliedfurther to the common antenna ANT.

[0026] The assumption in the case of FIG. 1 is that the base stationtransmitters can be tuned to frequencies belonging to three differentfrequency ranges, i.e. the B-range (the lowest frequency), the M-range(the intermediate frequency) and the T-range (the highest frequency).When the frequencies of a base station are changed, a frequency channelin a selected frequency range is given to the use of each transmitter.During manufacture of the base station summing network, conductors 4 to6 are so dimensioned that a given branch does not load the channel unitsof the other branches. This is achieved when the length of theconductors is D=λ/2, wherein λ is the wavelength at the M-frequency.Accordingly, the summing network is optimally tuned in the M-frequencyrange.

[0027] In order not to have to retune the summing network duringfrequency change, a compensation element 7 is added to the summingnetwork. In the embodiment of FIG. 1, compensation element 7 is composedof a conductor whose length corresponds to the total length ofconductors 4 to 6, and whose electrical characteristics correspond tothe electrical characteristics of conductors 4 to 6. In the case of FIG.1, compensation element 7 is arranged at distance L from the summingpoint. Distance L is so selected that L=λ/4+n*λ/2, wherein λ is thewavelength at the intermediate frequency of the frequency band of thebase station, and n is a positive integer or zero.

[0028] If base station transmitter frequencies are changed lower, i.e.to the B-range, signal wavelengths change such that the signalwavelength no longer corresponds to the length of conductors 4 to 6. Thecable length error caused by a frequency change affects summing point Pcausing it an inductive load. When moving to distance L from summingpoint P, the reactive impedances visible at the summing point take anopposite sign. This summing network impedance change that turns intocapacitance can be compensated for with compensation element 7, whichthus causes an inductive load of a corresponding magnitude.

[0029] Similarly, when base station transmitter frequencies are changedhigher, i.e. to the T-range, signal wavelengths change such that they nolonger correspond to the length of conductors 4 to 6. The conductorlength error caused by the frequency change affects summing point Pcausing it a capacitive load that changes into inductance when moving todistance L from summing point P. Since the load caused by thecompensation element corresponds to the load caused to the summing pointby the branches, this capacitive load compensates for the branchimpedance change that turned into inductance.

[0030] If compensation element 7 used as described above is a conductorwhose electrical characteristics and length correspond to the lengths ofconductors 4 to 6, with which band-pass filters 1 to 3 are coupled tothe summing point, then the solution simultaneously achieves temperaturecompensation for the summing network. This is because the changes causedby temperature changes to the electrical characteristics are balanced onthe different sides of the summing point.

[0031]FIG. 2 is a block diagram of a second preferred embodiment of thesumming network of the invention. The embodiment of FIG. 2 correspondsto the case of FIG. 1 in other respects, except that it uses a differentcompensation element. In the case of FIG. 2, the compensation elementused is a coil 9 and a capacitor 10, whose first poles are coupled to aconductor between summing point P and the antenna ANT, and second polesare grounded. Compensation element 8 thus serves as a resonator circuitfor the intermediate frequency of the frequency band used.

[0032]FIG. 3 is a block diagram of a third preferred embodiment of thesumming network of the invention. The embodiment of FIG. 3 alsocorresponds to the case of FIG. 1 in other respects, except that it usesa different compensation element. In the embodiment of FIG. 3,compensation element 11 comprises a summing element 12, e.g. similar tosumming point P. The actual signal passes directly through summingelement 12. In the case of FIG. 3, a short open stub 14 acting ascapacitance and a longer stub 13 acting as inductance are connected tosumming element 12. In practice, stubs 13 and 14 may be composed ofconductors of a given length and be dimensioned such that the conductorsare in parallel resonance at the M-frequency. At the B-frequency, thecoupling is visible as transverse inductance, and at the T-frequency astransverse capacitance.

[0033] The length of stubs 13 and 14 determines whether they act asinductance or capacitance. If the stub is closed at its end (grounded),it acts as inductance if its length is between (0 to λ/4)+n*λ/2, and ascapacitance if its length is (λ/4 to λ/2)+n*λ/2. If, again, the stub isopen (its end is not grounded), its acts as inductance if its length isbetween (λ/4 to λ/2)+n*λ/2,and as capacitance if its length is (0 toλ/4)+n*λ/2.

[0034] Instead of one stub acting as capacitance and one stub acting asinductance being coupled to summing element 12, several parallel stubsacting as capacitance and/or several parallel stubs acting as inductancemay be coupled thereto. In some cases, this provides a solution havingeven smaller losses than before.

[0035]FIG. 4 is a flow diagram of a first preferred embodiment of themethod of the invention.

[0036] In FIG. 4, in block A, the load caused to the summing point bythe summing network branches is determined. This may be accomplishede.g. by finding out the inductive reactance and capacitive reactancecaused by the branches.

[0037] In block B, a compensation element having a load corresponding tothat caused by the branches to the summing point is selected. A suitablecompensation element is e.g. a resonator whose transverse load isreactive and increases from the B-range to the M-range from inductive toinfinite, and decreases from infinite to capacitive when moving from theM-range to the T-range. At its simplest, a conductor may be selectedthat has the same corresponding length as the conductors connecting thepass-band filters of the branches to the summing point, and whoseelectric characteristics correspond to those of the conductorsconnecting the band-pass filters to the summing point.

[0038] In block C, the compensation element is coupled to the summingnetwork between the summing point and the antenna. In this case,distance L to the summing point is preferably selected so thatL=λ/4+n*λ/2, when λ is the wavelength and n is a positive integer orzero.

[0039]FIG. 5 is a block diagram of a fourth preferred embodiment of thesumming network of the invention. The embodiment of FIG. 5 largelycorresponds to that of FIG. 2. In the case of FIG. 5, however, thesumming network comprises two corresponding summing points P, to both ofwhich two branches are coupled. If the assumption is, by way of example,that FIG. 5 shows a summing network in transmission use, then summingpoints P combine signals produced by channel units TX1 to TX2 and TX3 toTX4, respectively, comprised by the branches.

[0040] In the case of FIG. 5, summing points P are not directly coupledto the antenna ANT, as is the case in FIG. 2 as regards summing point P.Instead, summing points P are coupled to the antenna ANT via a secondsumming point P2. In other words, the compensation elements 8 of thesumming points are arranged between summing points P and the secondsumming point P2. In addition, a second compensation element 15 isarranged between the second summing point P2 and the antenna ANT. Thissecond compensation element 15 compensates for the load caused to thesecond summing point P2 from the direction of the branches. The electriclength of the conductors connecting summing points P and P2 isλ/2+n*λ/2.

[0041] The second compensation element 15 is coupled at distanceL=λ/4+n*λ/2 from the second summing point P2, whereby λ is thewavelength at the intermediate frequency of the base station frequencyband, and n is a positive integer or zero. Although FIG. 5 describes, byway of example, that the second compensation element is composed of acombination of a coil and a capacitor, another type of compensationelement may also be used as the second compensation element instead ofthis alternative.

[0042]FIG. 6 is a block diagram of a fifth preferred embodiment of thesumming network of the invention. In the embodiment of FIG. 6, thesumming network branches comprise summing points P3, via which theband-pass filters of the branches are coupled to summing point P. Asdistinct from the case of FIG. 6, one or more channel units may also becoupled to summing point P directly via a channel unit-specificband-pass filter, i.e. without such a branch being coupled to summingpoint P via summing points P3.

[0043] The electrical length of the conductors connecting summing pointsP3 and P is λ/2+n*λ/2. A compensation element 16 is arranged betweensumming point P and the antenna ANT for compensating for the load causedto summing point P by the branches. The compensation element 16 is thusso dimensioned that it causes the summing point a load that correspondsto the total load caused to summing point P from the direction of thebranches and includes the load caused by both the conductors and summingpoints P3.

[0044] Although the description of FIG. 6 states by way of example thatcompensation element 16 is composed of a combination of a coil and acapacitor, another type of compensation element may also be used ascompensation element 16 instead of this alternative.

[0045] It is to be understood that the above specification and therelated figures are only intended to illustrate the present invention.Different variations and modifications of the invention are apparent tothose skilled in the art, without deviating from the scope and spirit ofthe invention disclosed in the attached claims.

1. A method of tuning a summing network, characterized by: determining the load caused to a summing point by summing network branches, selecting a compensation element whose load effect corresponds substantially to the load caused to the summing point by the summing network branches, and coupling said compensation element to the summing network between the summing point and an antenna.
 2. A method as claimed in claim 1, characterized by selecting a compensation element whose inductive reactance and capacitive reactance substantially correspond to the inductive reactance and capacitive reactance caused to the summing point by the branches.
 3. A method as claimed in claim 1 or 2, characterized by coupling said compensation element to the summing network between the summing point and the antenna at an electrical distance L from the summing point such that L=λ/4+n*λ/2, wherein λ is the wavelength at the intermediate frequency of the frequency band of the base station, and n is a positive integer or zero.
 4. A summing network comprising: a summing point (P) having interfaces for coupling summing network branches to the summing point, and an interface for coupling the summing point (P) to an antenna (ANT), and channel units (TX1 to TX3) arranged in the branches, and channel unit-specific band-pass filters (1 to 3), whose pass frequencies correspond to the frequency or frequencies of the corresponding channel unit, characterized in that a compensation element (7, 8, 11, 16) is coupled to summing network between the summing point (P) and the antenna (ANT), the load effect of said compensation element corresponding substantially to the load caused to the summing point (P) by the branches coupled to the summing point.
 5. A summing network as claimed in claim 4, characterized in that the inductive reactance and capacitive reactance of the compensation element (7, 8, 11, 16) correspond substantially to the inductive reactance and capacitive reactance caused to the summing point by the branches.
 6. A summing network as claimed in claim 4 or 5, characterized in that the compensation element (7, 8, 11, 16) is coupled between the summing point (P) and the antenna (ANT) at an electrical distance L from the summing point such that L=λ/4+n*λ/2, wherein λ is the wavelength at the intermediate frequency of the frequency band of the base station, and n is a positive integer or zero.
 7. A summing network as claimed in any one of claims 4 to 6, characterized in that the compensation element (7) is composed of a conductor whose length corresponds substantially to the total electrical length of conductors (4 to 6) connecting the band-pass filters to the summing point (P).
 8. A summing network as claimed in any one of claims 4 to 6, characterized in that the compensation element (8) is composed of a capacitor (10) and a coil (9) whose first poles are coupled to a connector between the summing point and the antenna, and whose second poles are grounded.
 9. A summing network as claimed in any one of claims 4 to 6, characterized in that the compensation element is composed of a resonator.
 10. A summing network as claimed in any one of claims 4 to 6, characterized in that the compensation element (11) is composed of a summing element (12) to whose interfaces are coupled at least one stub (13) acting as inductance and at least one stub (14) acting as capacitance, the stubs being dimensioned such that they are in resonance at the intermediate frequency of the base station's frequency band.
 11. A summing network as claimed in any one of claims 4 to 10, characterized in that said summing point (P) is coupled to said antenna (ANT) via a second summing point (P2), and that a second compensation element (15) is coupled between the second summing point (P2) and the antenna (ANT), the load of the second compensation element corresponding substantially to the load caused to the second summing point (P2) from the direction of the branches.
 12. A summing network as claimed in any one of claims 4 to 10, characterized in that the branches of the summing network comprise summing points (P3) via which the band-pass filters are coupled to said summing point (P). 